Diffusion transfer element and method of using same

ABSTRACT

An element with a base sheet having an organophilic surface, an hydrophilic, liquid permeable layer on the organophilic surface capable of becoming friable on reduction therein of metal ions to metal, which layer contains a metal ion reduction promoter is used in the formation of lithographic plates by diffusion transfer or electrolytic processes.

United States Patent Eikvar Oct. 21, 1975 [54] DIFFUSION TRANSFER ELEMENT AND 2,971,840- 2/1961 Haydn et a1. 96/29 METHOD OF USING SAME 3,165,458 1/1965 Harriman 204/18 3,245,784 4/1966 Stricklin 96/33 X Inventor: T Elkvar, p e ood, 3,385,701 5/1968 Ormsbee et a1 96/33 x [73] Assignee: Minnesota Mining and Manufacturing Company Paul Minn Primary Examiner-Roland Martin, Jr. Attorney, Agent, or FirmAlexander, Sell, Steldt & [22] Filed: Sept. 21, 1964 DeLaHunt 21 Appl. No: 397,986

[57] ABSTRACT 96/29 96/29 An element with a base sheet having an organophilic 96/33; 96/76 117/8; surface, an hydrophilic, liquid permeable layer on the 204/18 organophilic surface capable of becoming friable on 101/457; 101/463 reduction therein of metal ions to metal, which layer f 8 G030 contains a metal ion reduction promoter is used in the Fleld 0f Sealdl 1 formation of lithographic plates by diffusion transfer o electrolytic processes 5 References Cited 15 Claims, 2 Drawing Figures UNITED STATES PATENTS 2,774,667 12/1956 Land et a1 96/29 US. Patent 00:. 21, 1975 3,914,125

STAGE 4' I N VE N TOR 7&z5 Fwy/7p DIFFUSION TRANSFER ELEMENTAND METHOD OFUSING SAME This invention relates to lithographic printing and more particularly to a photolithographic-image receptor sheet and to the method for making same.

In lithography many processes have been used to define the image and non-image (background) areas on a lithographic plate to render those areas selectively ink-receptive and water-receptive (ink repellant), as the.case may be. Special types of lithographic plates have been designed for use with certain imaging .processes.,Thus, for example, many types of so-called direct image plates have been suggested and used for imaging by such. means as direct art work with pencil, typewriter, set type, etc. To a lesser extent electrically inscribable lithographic plates, electrostatic imagable lithographic plates, and such like, have been used.

In a field of lithography which has attained more or less separate status, ordinary or actinic light is employed as the imaging means. The broad area of lithography wherein an image is defined on a lithographic plate coated with a light sensitive material by exposing the plate to light through a stencil or transparency, is known as photolithography. Photolithography is related to photography in that, in both, light is used to define an image (often latent) within a light reactive material which is thereafter processed .by various chemical and/or physical means to produce, in the case of photography, a viewable image, and in the caseof photolithography a printable image (wherein, on a lithographic press, some areas will take ink and others will not).

The distinctive requirements for producing a printable image as opposed to a viewable image has led to two arts generally to pursue very different development programs. In fact, the stringent and very different surface requirements which must be metin a photolitho graphic plate have caused the photolithographic art to lag the photographic art by some years inrespect to ostensibly analogous developments. For example, lightsensitive photographic film which could be sold in light-proof containers, shipped, stored for weeks or months, and thereafter used, was available well over a decade prior to the time the first presensitized metal .IithographicplateS became available in the year 1950: see Jewett et al. U.S., Pat. No. 2,714,066. More pertinently, about to years ago the photographic art was able to produce photographs in a matter of seconds by the so-called silver salt diffusion transfer processes; (for example, see Land U.S. Pat., Nos. 2,698,237, 2,759,825 and 2,765,240) but to the present day, preparation for printing of photolithographic plates as available in c ommerce, has been comparatively timeconsuming usually requiring several minutes to complete the operation.

The silver salt diffusion transfer process was not applicable per se to the photolithographic artin that the silver metal, when transferred to a background. material, did not alter the wetting properties of the background material as required for lithographic printing (although it did provide a satisfactory viewable, i.e., hotographic, image).

Attempts have been made to. adopt this rapid photo graphic process for photolithographic'printing by transferring a silver image to a hydrophilic surface, and proicessing the image to make it ink receptive. For example, see Lassig et al. U.S. Pat. Nos. 3,083,087 and 3,063,837. However, these attempts apparently have seen little or no commercial success. The present invention, insofar asl am aware, employs an entirely new concept in photolithography, to provide novel lithographic plates utilizing very rapid photographic silver salt diffusion transfer or similar metal-transferring processes to define the image areas of a lithographic plate without using the transferred image per se or any modification of it as the ink receptive lithographic image.

My invention will be readily understood upon a consideration of the description herein taken as a whole, including the accompanying drawing, in which:

FIG. 1 shows one embodiment of my invention in broken-away edge view at various stages of its manufacture, and also illustrating light-exposure of the plate, processing and development of yield a lithographic plate. Thus, under Stage A is shown a base sheet 10 having an organophilic surface. Coated over the orgarophilic surface is a hydrophilic layer 11, which is firmly adherently bonded to the organophilic surface. The hydrophilic layer is liquid permeable throughout its depth, and is characterized in becoming weak or friable upon the in situ reduction therein of metal ions, for example silver ions, to their metallic state and subsequent removal of the reduction product. In this embodiment, the hydrophilic layer contains at and immediately adjacent the interface between the hydrophilic layer and the organophilic surface, an interfacial area designated as 12, of a metal-ion reduction-promoter.

Under Stage B is shown the structure at Stage A to which has been added a coating 13 of a high contrast photographic emulsion. Actually the emulsion layer 13 may be a conventional preformed photographic film which has been light-exposed to an image (but not developed), or it may be an adherent cast light-sensitive emulsion coating. The latter variant is preferred.

Under Stage C is shown the structure at Stage B after light-exposure thereof to an image. In the light exposed areas the silver compound in the emulsion layer 13 is converted into a latent reduced state, in areas 13a, while remaining unexposed and still light-sensitive in areas 13b.

Under Stage D is shown the structure at Stage C after it has been immersed in a developer solution. The silver compound in the light-struck areas 13a is now reduced to the metallic state and is a visible black color. The soluble silver halide previously present in the areas 13b has transferred or diffused along with the developing solution through the layer 11 and into contact with the metal ion reduction promoter at the interfacial areas 12b where the silver is reduced to the metallic state. Actually, the reduction of the silver in the interfacial areas 12b ordinarily takes place a few seconds later than in the light-exposed areas of the emulsion layer 13a, so that, as the plate is developed an image (which is negative with respect to the original) will first appear in black in areas 13a, following which the entire viewable surface of the plate will appear black as the silver isreduced to the metallic state in the interfacial areas Under Stage E is shown the partially processed plate at Stage D after the plate has been further processed and rubbed down lightly. As will be more fully described hereinafter, the entire emulsion layer 13 has been removed, and the hydrophilic layer in areas 11b (as defined by and including the silver image of areas 12b) has also been removed to lay bare the underlying organophilic surface areas b of the base sheet. The areas 11a (including the areas 12a) of the hydrophilic surface remain firmly bonded to the base sheet to make up the non-printing background areas of the plate whereas the exposed areas 10b of the organophilic surface make up the ink-receptive printing image areas of the plate.

FIG. 2 shows an alternative embodiment of the present invention in broken-away edge view of the various stages of process to form a lighographic plate. As shown under Stage A it comprises a sheet 20 having a conductive metal surface 20 over which is coated a firmly bonded organophilic layer 21 which is photoconductive, e.g., which contains photoconductive material. Thereover, and firmly bonded thereto, is a layer of liquid permeable hydrophilic material 22. The photoconductive material contained in layer 21 serves as the metal ion reduction promoter. Thus, under Stage B the construction of Stage A is shown after it has been lightexposed to an image. The areas 21a of, the organophilic layer at the interface between such layer and the overlying hydrophilic layer (and elsewhere throughout its thickness) have been rendered electrically conductive by light exposure. Under Stage C the structure of Stage B is shown after the plate has been connected to an electric potential (with metal surface 20 and electrically conductive area 21a serving as the cathode) in the presence of an electrolyte containing reducible metal ions (e.g., silver ions). After a time, in response to the electroconductive forces, the silver ions come into contact with and deposit upon the cathode surface of areas 21a and become reduced to their metallic state ene resin and an additional one-half mil polypropylene resin, (said support sheet being available'from Riverside Paper Corporation, Appleton, Wisconsin) was corona discharge treated for adhesion. The treated sup- H port sheet was coated with an 8 micron (0.00032inch thick organophilic dry layer from a solution containing 48% solids by weight of Union Carbides Vinylite VAGH vinyl chloride-acetate resin and American Cyanamids Unitane OR350 (titaniumdioxide) pigment in a ratio of 2.65 parts Ti0 to 1 part by weight VAGI-I resin. The pigment previously had been dispersed in the to yield a black image 23 which is negative in respect to the original. Under Stage D is shown the structure of Stage C after further processing to remove the deposited silver of image 23 and the overlying hydrophilic layer at 22a to lay bare the underlying organophilic surface in areas 21a and thereby to define an ink receptive printing image. The remaining areas 22b of the hydrophilic layer 22 remain firmly bonded to define the hydrophilic background areas of the plate.

In each of the above figures, the letter b is used to denote areas corresponding to the dark areas of the original and small letter a is used to denote areas corresponding to the light areas of the original. It will thus be noted that in the embodiment shown in FIG. 1, the resulting plate has b areas as the printing areas and the a as the background areas, i.e., is a positive acting plate; whereas the embodiment shown in FIG. 2 has the b areas as the background areas and the a areas as the printing areas, i.e., is a negative acting plate.

All dimensions in the drawing are greatly exaggerated for clarity of illustration, and are not meant to depict relative thicknesses of the several layers. However, it will be observed, in connection with each of the figures of the drawing, that within the interfacial areas of the hydrophilic liquid permeable layer and the underlying organophilic layer an interfacial area is defined which contains a metal ion reduction promoter.

My invention will now be more specifically described with the aid of the following examples, presented in order to illustrate the invention and not to limit it.

EXAMPLE 1 A support sheet composed of bleached paper coated with one-half mil (0.0005 inch low density polyethylsolution by ball milling for. 48 hours and the pigmented solution in methyl ethyl ketone solvent was coated on the support sheet and dried at 150F. for 5 minutes.

Over the organophilic coating was applied a hydrophilic silica layer, in two parts. The first part or sub layer was comprised of 0.1 micron thick dry coatingof collodial silica and colloidalsilver whose formulation was:

The sub-layer was dried for 20 seconds at 150F. and" then overcoated with the second part comprised of a 0.2-0.8 micron thick dry' coating of the following 001-: loidal silica formulation:

Ingredient Amount Ludox" AM colloidal silica,

30% solids (DuPont) 800 grams "Teepol" (Shell Chemical Company) anionic sulphate wetting agent 4 milliliters This second part or outer layer was dried for 20 seconds at l50F. A high contrast,'chlorobromide photographic emulsion, e.g., having a chloride/bromide ratio 5 of 2: I and a gelatin/silver ratio of 1.2:] was then coated over the silica layer. This was coated at a silver coating weight of 25 milligrams per square decimeter.

A 10 inch X 16 inch sheet of the above material was placed on the vacuum back of a Robertson 320 process camera fitted with an image reversal lens. The plate was then exposed to a right reading, positive, line copy original at a 32 f stop for 8 seconds. The exposed plate was then immersed for 30 seconds at 72F. in a tray of diffusion transfer developer with the following composition:

Ingredient Amount deionized water 1000 milliliters sodium sulfite grams hydroquinone 35 grams sodium thiosulfate 15 grams sodium hydroxide 28.5 grams potassium bromide 2.5 grams 0.5% benzotriazole in water milliliters This diffusion transfer developer differs from a conventional photographic developer by the addition of sodium thiosulfate which acts as a diffusion transfer agent, dissolving and forming a complex with silver halide, but not with silver in either its light-reduced or developer reduced form. Following immersion in the developer a silver negative image appeared in the light struck areas, and shortly thereafter a silver positive image was formed in the hydrophilic layer. The emulsion layer was then removed by running tap water at a temperature of about 110C. over the plate for about 20 seconds to lay bare the underlying hydrophilic surface containing the positive silver image. The plate was then immersed for 5 seconds at 75F. in a bleach solution of the following composition:

This bleaching step leached out the silver metal from the hydrophilic layer, and the remaining bleach solution was removed by water rinsing the plate. The plate was then gently swabbed during rinsing with a cotton pad which cleanly removed the silica from the image areas to lay bare the underlying ink receptive organophilic surface. The remaining silica layer of the nonimage areas remained firmly bonded to the organophilic surface to provide the ink-repellant background areas of the plate. The plate was then ready for the press and upon test running, several hundred faithful copiess were produced.

Although we do not wish to belimited by the following theory, it is believed that the properties assumed by the image area of the silica which enables its easy removal to bare the underlying organophilic surface, is the result of properly controlling the reduction of the silver halide within the hydrophilic layer. Initially (and finally in the background areas) the hydrophilic layer is necessarily relatively tough and securely bonded to the organophilic layer. The bond between the silica and the organophilic layer within the image areas must be ruptured if the silica in said image areas is to be preferentially removed by light rubbing. This rupturing is believed to be accomplished by providing for penetration of the silver halide solution to the interface between layers and 11 to there become reduced by the silver reduction nuclei. The forming of the silver metal crystals causes the desired rupturing of the silica bond and when the silver is removed by the bleaching solution, the remaining loosely bonded particles of the silica within the image areas can be cleanly broken away and easily removed to lay bare the underlying organophilic surface, without disturbing the silica of the background areas.

Control of the silver reduction process is achieved in the above example by limiting the presence of the silver reduction nuclei to the area immediately adjacent to the interface. However it will be understood that other means of control are available an example of which is set forth as follows:

EXAMPLE 2 The same support sheet and pigmented vinyl resin coating was used as in Example 1. The support sheet with coating was further overcoated with a 0.2-0.8 micron thick dry layer of colloidal silica and colloidal silver which is coated out of an aqueous solution and dried 20 seconds at F., and whose formulation is:

The same high contrast photographic emulsion as in Example 1 was'coated over the hydrophilic layer and a 10 inch X 1 6 inch sheet of the composite material was exposed and processed in the same manner as described for Example 1. This resulted in the removal of the hydrophilic silica layer in the non-light-struck (image) areas thereby to lay bare the underlying organophilic printing surface. The plate was then ready for the press and upon test running produced several hundred faithful copies.

It will be observed that in the plate structure of the present Example, the silver reduction nuclei, while being present at the interface of the hydrophilic layer and the organophilic layer, are not concentrated only at the interface, but extend throughout the hydrophilic layer. In this type of construction, wherein silver reduction nuclei are dispersed throughout the hydrophilic layer, it has been found to be highly preferable to include also in the hydrophilic layer the glycerine inhibitor to inhibit premature reduction of the metal ions as they transfer throughout they hydrophilic layer. Such inhibitor serves to insure that a substantial reduction of the metal ions takes place at the interface to effect an adequate rupturing of the bond between the hydrophilic and organophilic layers whereby the hydrophilic layer will be broken-away cleanly in the area defined in the printing image. While the inhibitor does not seem to be vitally necessary where the plate is to be used immediately or quite soon after manufacture, it has been found that upon storage of the plate for weeks or months before use, the hydrophilic silica layer tends to change in character so as prematurely to cause reduction of the silver ions to their metallic state resulting in too few of the silver ions being reduced at the interface. When this occurs, it is difficult to process the plate so that the hydrophilic material is cleanly and thoroughly removed in the image areas, resulting in a blinded image. The presence of the glycerin in the present Example serves adequately to inhibit premature reduction of the silver, even after the plate has been stored for weeks or months before use.

The glycerin employed as the stabilizing inhibitor in the present Example will be seen to be a hygroscopic material, i.e., a humectant. Other humectants exhibit similar properties, and find use in connection with the present invention.

In the above examples, the latent image was formed by light-exposure of a photographic emulsion that was coated over the silica. This type of embodiment of the invention provides a single sheet photolithographic plate which has certain advantages over a multiple sheet construction wherein the photographic emulsion is incorporated in separate sheeting. For example, the single sheet construction is easier to package, it insures intimate contact between the emulsion and the hydrophilic layer, it enables the manufacturer of the plate to maintain control of the type of emulsion that is used, i.e., so that the most suitable type of photographic emulsion for a particular receptor sheet is used, and a savings in cost is realized in that it only requires a single backing, whereas a separate emulsion layer requires a separate backing. Also, with the photographic emulsion already applied to the plate, the steps which must be performed by a plate maker or lithographer are reduced to where all he need do is remove the plate from its light-proof container, and expose and process it.

It will be understood, however, that the present invention is also applicable to a two sheet. structure wherein the photographic emulsion is omitted from the plate and carried by a separate film carrier, e.g., a conventional photographic film. Thus the latent image can be formed by exposure on the photographic film, and development is effected in the plate structure as illustrated in the following example.

EXAMPLE 3 A support sheet composed of a 5 mil (0.005 inch) A type Mylar" film from DuPont was coated with 2.0 grams per square foot dry weight of a solution containing'48% solids by weight of Union Carbides Vinylite VAGH resin and American Cyanamids Unitane OR350 (titanium dioxide) pigment in a ratio of 2.65 parts TiO to 1 part by weight VAGl-I vinyl chlorideacetate resin, prepared as indicated in Example 1. This pigmented solution is methyl ethyl ketone solvent was coated on the Mylar base material and dried at 150F. for 5 minutes. Thus coated support sheet was further overcoated from aqueous solution with a 0.2-0.8 micron thick dry layer of colloidal silica and colloidal silver of the following formulation:

at l0% solids in water After coating the plate was dried at 150F. for 20 seconds.

A inch X 16 inch sheet of photographic film (Dinoline Acetate Ortho) was exposed without reversal to a right reading positive, line copy original and then wetted with the developer solution of the preceding example. Immediately following the wetting of the film, the emulsion side thereof was positioned over and in intimate contact with the receptor sheet. Thus the development was carried out while the emulsion of the film overlay the silica of the receptor sheet, per much the same manner as the preceding example. Following development the two-sheets were peeled apart and the receptor sheet bleached and rinsed and rubbed lightly in the same manner as described in Example 1. The plate was then ready for the press and upon test running produced several thousand faithful copies. (The EXAMPLE 4 A support sheet comprised of a 69 lb. Crocker Hamilton paper was laminated to a 0.3 mil aluminumwhich in turn was overcoated with 2.5 to 3.0 grams per square foot (dry weight) of an organophilic photoconductive layer of the following formulation:

Ingredient Amount Pliolite S-7 polystyrene butadiene resin (Goodyear) 19.2 grams polystyrene (Dow Chemical Company) PSII l2.8 grams TiO pigment (DuPont) FFCR 19.6 grams ZnO pigment (New Jersey Zinc) USP12 134.0 grams toluene solvent 206.4 grams methanol solvent 8.0 grams Phosphine R dye 0.044 grams The organophilic layer was further overcoated with 0.5 grams per-squarc foot (dry weight) of a hydrophilic layer of the following formulation:

Ingredient Amount Colloidal silica Nalcoag I034A (Nalco Chemical Company) 60 grams water 33 grams methanol 7 grams The plate was exposed for 30 seconds by projecting through a negative microfilm at a distance of 4 feet using a projector with a 400 w. 65 v. bulb. The plate was then developed by electroplating a silver image in the light struck areas. The anode consisted of a mesh of silver wire, and the electrolyte comprised a solution of 0.1 N silver nitrate. The aluminum laminate and the light exposed, conductive zinc oxide image areasacted as a cathode and a layer of sponge held the anode and cathode at a distance of approximately one-fourth inch.

bond between the hydrophilic and organophilic layers had been ruptured in the same way during formation and dissolution of the silver metal image, yielding .in

like manner a lithographic plate ready for the press. Upon test running, several hundred faithful copies were produced.

It it understood that the above description is directed to the examples for illustrating the present invention and that they do not limit the scope of the invention which is determined by the following claims:

What is claimed is: i 1. A sheet structure, suitable forprinting photolithographically, comprising a base sheet having an organophilic surface, a tough hydrophilic layer coated over andfirmly bonded to said organophilic surface, said hydrophilic layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, and a metal-ion reduction-promoter disposed at the inner surface of said hydrophilic layer, said hydrophilic layer being stabilized against undue reduction of metal ions passing therethrough prior to contacting the reduction-promoter at said inner surface, said hydrophilic layer, upon transfer therethrough in certain areas of metallic ions tosaid inner surface and into contact with said reduction promoter to become reduced to the metallic state, being readily broken cleanly away in said areas of such transfer to lay bare the underlying organophilic surface and define a printing image, while remaining hydrophilic ,and firmly bonded in the remaining areas.

2. A sheet structure, suitable for printing photolithographically, comprising a base sheet having an organophilic surface, a though hydrophilic layer coated over and firmly bonded to said organophilic surface, said hydrophilic layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, a metal-ion reduction-promoter within said hydrophilic layer adjacent the inner surface thereof, and an inhibitor dispersed insaid hydrophilic layer to inhibit undue reduction of metal ions passing therethrough prior to contacting the reduction-promoter at said inner surface, said reduction-promoter upon being contacted by metal ions transferred through the hydrophilic layer in certain areas thereof, effectively reducing the metal ions to their metallic state to rupture the bond at the interface between the hydrophilic and organophilic layers and provide easy removal of the hydrophilic layer within said certain areas to lay bare the underlying organophilic surface, thereby defining a printing plate having a printing image in said certain areas, with the remaining areas, coated with the firmly bonded remaining areas of the hydrophilic layer, forming the background.

3. A sheet suitable for printing photolithographically as defined in claim 2 wherein said inhibitor comprises a humectant to maintain the hydrophilic layer sufficiently liquid permeable to provide rapid transfer of the metal ions through the hydrophilic layer to its inner surface prior to the reduction of the metal ions to the metallic state.

4. A sheet structure suitable for printing photolithographically, comprising a base sheet having an organophilic surface, a tough hydrophilic layer coated over and firmly bonded to said organophilic surface, said hydrophilic layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, said hydrophilic layer containing a metal-ion reduction promoter substantially only at the interface between said hydrophilic and organophilic layers, said reduction promoter upon being contacted by metal ions transferred through the hydrophilic layer in certain areas thereof, effectively reducing the metal-ions to their metallic state to permit rupture of the bond at the interface between the hydrophilic and organophilic layers and provide easy removal of the hydrophilic layer within said certain areas to lay bare the underlying organophilic surface, thereby defining a printing plate having a printing image in said certain areas, with the remaining areas, coated with the firmly bonded remaining areas of the hydrophilic layer, forming the background.

5. A sheet structure suitable for printing photolithographically, comprising a base sheet having an organophilic silica layer coated over and firmly bonded to said organophilic surface, said silica layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of silver ions to their metallic state and subsequent removal of the reduction product, a light-sensitive high contrast photographic emulsion coating lying in contact with the silica layer, and silver reduction nuclei disposed at the inner surface of said silica layer, said silica layer being stabilized against undue reduction of silver ions passing therethrough prior to contacting the silver reduction nuclei at said inner surface and, upon processing of the sheet to expose certain areas of the photographic emulsion and to transfer silver halide from the unexposed areas thereof through the hydrophilic layer to its inner surface to there become 'reduced to silver metal, being readily broken cleanly away in said unexposed areas to lay bare the underlying organophilic surface and define a printing image, while remaining hydrophilic and firmly bonded in the remaining areas.

6. A method for developing a photolithographic printing plate having a liquid permeable hydrophilic layer firmly bonded to an organophilic surface of a base layer, said hydrophilic layer being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, including transferring a metal-ion bearing liquid through the hydrophilic layer in certain image areas to the interface between the two layers, reducing the metal ions to their metallic state at the interface within said areas to rupture the bond between the layers and fracture the overlying hydrophilic layer, treating the plate with a bleaching and leaching solution to dissolve and leach out the metal, and lightly rubbing away the remaining fractured portion of the hydrophilic layer to lay bare the underlying organophilic surface.

7. A metal of selectively imaging a substrate overcoated with a friable liquid permeable hydrophilic material comprising passing metal ions through the depth of said layer of hydrophilic material and reducing said metal ions to their metallic state at the interface of said hydrophilic layer and said substrate in certain areas, avoiding reduction of said metal ions in the remaining areas of said plate to define an image.

8. A process for printing photolithographically by utilization of a sheet structure, said sheet structure comprising a base sheet having a conductive metal surface, an unexposed organophilic photoconductive layer that becomes electrically conductive when exposed to light coated over the metal surface of the base sheet, a tough, liquid permeable hydrophilic layer firmly bonded to the photoconductive meterial, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, the process comprising exposing said photoconductive material to light in certain image areas, subjecting said sheet to an electromotive force in an electrolytic environment wherein said conductive metal is connected as the cathode thereby causing transfer of the metal ions of an electrolyte through the hydrophilic layer, depositing and reducing said metal ions at said interface within said image areas, and rupturing the bond at said interface removing said hydrophilic layer within said certain areas and laying bare the underlying organophilic sur face to define a printing plate having a printing image in said certain areas and coated with the firmly bonded remaining areas of the hydrophilic layer to form a background.

9. A sheet structure suitable for photolithographic printing comprising a base sheet having a hydrophobic surface, a nucleated hydrophilic coating overlying the non-image areas of said surface, said hydrophobic surface being exposed in the image areas, said coating being characterized by the ability to receive silver halide by diffusion transfer in image areas when an exposed negative is brought into contact with said coating in the presence of a developing solution for the negative, metallic silver being formed in said coating due to the reduction of said halide resulting from reaction with said developing solution thereby enabling the removal of said coating in said image areas.

10. A sheet structure suitable for photolithographic printing in accordance with claim 9 wherein said coating comprises a thin, relatively porous film wherein the formation of metallic silver in the image areas during the developing operates to physically weaken the film in the image areas to thereby enable removal of said film from the image areas by washing of the sheet with an aqueous solution.

11. A sheet structure suitable for photolithographic printing in accordance with claim 9 wherein said coating comprises a thin, relatively porous water insoluble film adapted to be removed from image areas on the sheet by means of a bleaching solution; 12. A sheet structure suitable for photolithographi printing in accordance with claim 9 wherein said hydrophobic surface is provided by means of a coating disposed over the surface of a base sheet.

13. A method for the formation of a sheet structure suitable for photolithographic printing comprising the steps of providing a base sheet having an ink receptive hydrophobic surface, applying a coating over said sur- 7 face comprising a nucleated hydrophilic material, said coating being characterized by the ability to receive silver halide by diffusion transfer when an exposed negative is brought into contact with said coating in the 14. A method in accordance with claim 13 wherein: said coating comprises a thin, relatively porous film, and wherein the formation of metallic silver in the,

image areas during the developing operates to physically weaken the film in the image areas to thereby enable removal of said film from the image areas by washing of the sheet with an aqueous solution.

15. A method for the formation of a sheet structure suitable for photolithographic printing in accordance with claim 13 wherein said coating comprises a thin, relatively porous water insoluble film adapted to be removed from image areas on the sheet by means of a.

bleaching solution.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,91 r,125-

DATED October 21, 1975 |NVENTOR(S) I Tore Eikvar it is certified that error appears in the above-identified patent and that said Letters Patent 0 are hereby corrected as shown below:

Col. 1, line 35 should read "led the two arts" instead of "led to two arts".

Col. 2, line 17 should read "to yield" instead of "of yield".

Col. 6, line 37 should read "the hydrophilic layer" instead of "they hydrophilic layer".

Col. 10, line 13 should read "organophilic surface, a tough hydrophilic silica layer coated" instead of "organophilic layer coated".

Col. 10, line 52 "metal" should read "method".

0 r Signed and Scaled this twentyth D ay Of January 1 9 76 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflatents and Trademarks 

1. A SHEET STRUCTURE, SUITABLE FOR PRINTING PHOTOLITHOGRAPHICALLY, COMPRISING A BASE SHEET HAVING AN ORGANOPHILIC SURFACE, A TOUGH HYDROPHILIC LAYER COATED OVER AND FIRMYL BONDED TO SAID ORGANOPHILIC SURFACE, SAID HYDROPHILIC LAYER BEING LIQUID PERMEABLE THROUGHOUT ITS DEPTH, AND BEING CHARACTERIZED IN BECOMING FERIABLE UPON THE IN SITU REDUCTION THEREIN OF METAL IONS TO THEIR METALLIC STATE AND SUBSEQUENT REMOVAL OF THE REDUCTION PRODUCT, AND A METAL-ION REDUCTION-PROMOTER DISPOSED AT THE INNER SURFACE OF SAID HYDROPHILIC LAYER, SAID HYDROPHILIC LAYER BEING STABILIZED AGAINST UNDUE REDUCTION OF METAL IONS PASSING THERETHROUGH PRIOR TO CONTACTING THE REDUCTION-PROMOTER AT SAID INNER SURFACE, SAID HYDROPHILIC LAYER, UPON TRANSFER THERETHROUGH IN CERTAIN AREAS OF METALLIC IONS TO SAID INNER SURFACE AND INTO CONTACT WITH SAID REDUCTION PROMOTER TO BECOME REDUCED TO THE METALLIC STATE, BEING READILY BROKEN CLEANLY AWAY IN SAID AREAS OF SUCH TRANSFER TO LAY BARE THE UNDERLYING ORGANOPHILIC SURFACE AND DEFINE A PRINTING IMAGE, WHILE REMAINING HYDROPHILIC AND FIRMYL BONDED IN THE REMAINING AREAS.
 2. A sheet structure, suitable for printing photolithographically, comprising a base sheet having an organophilic surface, a though hydrophilic layer coated over and firmly bonded to said organophilic surface, said hydrophilic layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, a metal-ion reduction-promoter within said hydrophilic layer adjacent the inner surface thereof, and an inhibitor dispersed in said hydrophilic layer to inhibit undue reduction of metal ions passing therethrough prior to contacting the reduction-promoter at said inner surface, said reduction-promoter upon being contacted by metal ions transferred through the hydrophilic layer in certain areas thereof, effectively reducing the metal ions to their metallic state to rupture the bond at the interface between the hydrophilic and organophilic layers and provide easy removal of the hydrophilic layer within said certain areas to lay bare the underlying organophilic surface, thereby defining a printing plate having a printing image in said certain areas, with the remaining areas, coated with the firmly bonded remaining areas of the hydrophilic layer, forming the background.
 3. A sheet suitable for printing photolithographically as defined in claim 2 wherein said inhibitor comprises a humectant to maintain the hydrophilic layer sufficiently liquid permeable to provide rapid transfer of the metal ions through the hydrophilic layer to its inner surface prior to the reduction of the metal ions to the metallic state.
 4. A sheet structure suitable for printing photolithographically, comprising a base sheet having an organophilic surface, a tough hydrophilic layer coated over and firmly bonded to said organophilic surface, said hydrophilic layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, said hydrophilic layer containing a metal-ion reduction promoter substantially only at the interface between said hydrophilic and organophilic layers, said reduction promoter upon being contacted by metal ions transferred through the hydrophilic layer in certain areas thereof, effectively reducing the metal-ions to their metallic state to permit rupture of the bond at the interface between the hydrophilic and organophilic layers and provide easy removal of the hydrophilic layer within said certain areas to lay bare the underlying organophilic surface, thereby defining a printing plate having a printing image in said certain areas, with the remaining areas, coated with the firmly bonded remaining areas of the hydrophilic layer, forming the background.
 5. A sheet structure suitable for printing photolithographically, comprising a base sheet having an organophilic silica laYer coated over and firmly bonded to said organophilic surface, said silica layer being liquid permeable throughout its depth, and being characterized in becoming friable upon the in situ reduction therein of silver ions to their metallic state and subsequent removal of the reduction product, a light-sensitive high contrast photographic emulsion coating lying in contact with the silica layer, and silver reduction nuclei disposed at the inner surface of said silica layer, said silica layer being stabilized against undue reduction of silver ions passing therethrough prior to contacting the silver reduction nuclei at said inner surface and, upon processing of the sheet to expose certain areas of the photographic emulsion and to transfer silver halide from the unexposed areas thereof through the hydrophilic layer to its inner surface to there become reduced to silver metal, being readily broken cleanly away in said unexposed areas to lay bare the underlying organophilic surface and define a printing image, while remaining hydrophilic and firmly bonded in the remaining areas.
 6. A method for developing a photolithographic printing plate having a liquid permeable hydrophilic layer firmly bonded to an organophilic surface of a base layer, said hydrophilic layer being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, including transferring a metal-ion bearing liquid through the hydrophilic layer in certain image areas to the interface between the two layers, reducing the metal ions to their metallic state at the interface within said areas to rupture the bond between the layers and fracture the overlying hydrophilic layer, treating the plate with a bleaching and leaching solution to dissolve and leach out the metal, and lightly rubbing away the remaining fractured portion of the hydrophilic layer to lay bare the underlying organophilic surface.
 7. A metal of selectively imaging a substrate overcoated with a friable liquid permeable hydrophilic material comprising passing metal ions through the depth of said layer of hydrophilic material and reducing said metal ions to their metallic state at the interface of said hydrophilic layer and said substrate in certain areas, avoiding reduction of said metal ions in the remaining areas of said plate to define an image.
 8. A process for printing photolithographically by utilization of a sheet structure, said sheet structure comprising a base sheet having a conductive metal surface, an unexposed organophilic photoconductive layer that becomes electrically conductive when exposed to light coated over the metal surface of the base sheet, a tough, liquid permeable hydrophilic layer firmly bonded to the photoconductive meterial, and being characterized in becoming friable upon the in situ reduction therein of metal ions to their metallic state and subsequent removal of the reduction product, the process comprising exposing said photoconductive material to light in certain image areas, subjecting said sheet to an electromotive force in an electrolytic environment wherein said conductive metal is connected as the cathode thereby causing transfer of the metal ions of an electrolyte through the hydrophilic layer, depositing and reducing said metal ions at said interface within said image areas, and rupturing the bond at said interface removing said hydrophilic layer within said certain areas and laying bare the underlying organophilic surface to define a printing plate having a printing image in said certain areas and coated with the firmly bonded remaining areas of the hydrophilic layer to form a background.
 9. A sheet structure suitable for photolithographic printing comprising a base sheet having a hydrophobic surface, a nucleated hydrophilic coating overlying the non-image areas of said surface, said hydrophobic surface being exposed in the image areas, said coating being characterized by the ability to receive silver halide by diffusion transfer in image areas when an exposed negative is brought into contact with said coating in the presence of a developing solution for the negative, metallic silver being formed in said coating due to the reduction of said halide resulting from reaction with said developing solution thereby enabling the removal of said coating in said image areas.
 10. A sheet structure suitable for photolithographic printing in accordance with claim 9 wherein said coating comprises a thin, relatively porous film wherein the formation of metallic silver in the image areas during the developing operates to physically weaken the film in the image areas to thereby enable removal of said film from the image areas by washing of the sheet with an aqueous solution.
 11. A sheet structure suitable for photolithographic printing in accordance with claim 9 wherein said coating comprises a thin, relatively porous water insoluble film adapted to be removed from image areas on the sheet by means of a bleaching solution.
 12. A sheet structure suitable for photolithographic printing in accordance with claim 9 wherein said hydrophobic surface is provided by means of a coating disposed over the surface of a base sheet.
 13. A method for the formation of a sheet structure suitable for photolithographic printing comprising the steps of providing a base sheet having an ink receptive hydrophobic surface, applying a coating over said surface comprising a nucleated hydrophilic material, said coating being characterized by the ability to receive silver halide by diffusion transfer when an exposed negative is brought into contact with said coating in the presence of a developing solution for the negative, said transfer taking place in positive image areas on said sheet and metallic silver being produced in said positive image areas, and removing said coating from said sheet in said image areas to expose the hydrophobic ink receptive surface in said image areas.
 14. A method in accordance with claim 13 wherein said coating comprises a thin, relatively porous film, and wherein the formation of metallic silver in the image areas during the developing operates to physically weaken the film in the image areas to thereby enable removal of said film from the image areas by washing of the sheet with an aqueous solution.
 15. A method for the formation of a sheet structure suitable for photolithographic printing in accordance with claim 13 wherein said coating comprises a thin, relatively porous water insoluble film adapted to be removed from image areas on the sheet by means of a bleaching solution. 